The present invention relates generally to improved low-cost alternating current (AC) light emitting diode (LED) drivers. More particularly, the present invention relates to apparatuses, systems, and methods for providing an AC LED driver with capacitive switching.
LED lighting is growing in popularity due to decreasing costs and long life compared to incandescent lighting and fluorescent lighting. LED lighting can also be dimmed without impairing the useful life of the LED light source.
Typically, an LED driver is a switching power supply, which means that a DC-DC converter must be used to regulate the LED load current. But the cost of switching power supply type LED driver is high, so it's desirable to minimize LED driver cost as much as possible. For this reason, directly-driven AC line LED drivers have emerged.
AC LED drivers have numerous advantages relating to the freedom from using DC-DC power converter technology and the extremely low cost. The AC LED driver simply drives the LEDs with AC input line power supply. However, most of AC LED drivers have complicated current control requirements and can't effectively solve the turn-on inrush current problem, which dramatically affects an LED's lifespan. As such, a low cost AC LED driver with inherent current limiting capability and with simple control is very desirable to maintain LED lamp competitiveness.
An LED driver with a capacitor limiting output current is very simple and cost effective for low current (e.g., 10 mA-150 mA) type LED applications. For example, FIG. 1 illustrates a system 100 having a simple LED driver 110 with only a capacitor to control the LED load current for a load 120. The LED driver 110 of system 100 has only a current limiting capacitor, C_I_limit, a bridge rectifier BR comprised of rectifying diodes D1-D4, and an output capacitor C_out. If the impedance of the current limiting capacitor C_I_limit is much greater than the load impedance, the LED load current can be set by C_I_limit as:
                    I_LED        =                                            2              ×                              2                                      π                    ×                                    V_in              ⁢              _ac                                      1                              2                ×                π                ×                                  f                  line                                ×                                  C                                      I                    ⁢                                                                                  ⁢                    _                    ⁢                                                                                  ⁢                    limit                                                                                                          Equation        ⁢                                  ⁢        1            
One major drawback of this solution is that the power factor is almost 0 since the capacitor impedance dominates the load. As a result, the input current will always be leading the input voltage by 90 degrees.